A collaborative team of researchers from the University of California, Berkeley, the Georgia Institute of Technology, and Ajou University in South Korea has revealed that the unique fan-like propellers of Rhagovelia water striders —which allow them to glide across fast-moving streams—open and close passively, like a paintbrush, ten times faster than the blink of an eye. Inspired by this biological innovation, the team developed a revolutionary insect-scale robot that incorporates engineered self-morphing fans that mimic the agile movements of Rhagovelia bugs. This study highlights how form and function of a biological adaptation shaped by natural selection, can enhance the locomotion and endurance of both water striders and bioengineered robots without incurring additional energy costs.
An automatic fan enhances interfacial motion
Rhagovelia water striders are unique among water striders because these millimeter-sized semiaquatic insects use specialized fan-like structures on their propulsion legs that enable rapid turns and bursts of speed.
"I was intrigued the first time I saw ripple bugs while working as a postdoc at Kennesaw State University during the pandemic." said Victor Ortega-Jimenez an integrative biologist now at the University of California, Berkeley, a lead author of the study. Ortega-Jimenez had previously studied the jumping performance of large Gerridae water striders from unsteady waters, but Rhahovelia bugs were different. "These tiny insects were skimming and turning so rapidly across the surface of turbulent streams that they resembled flying insects. How do they do it? That question stayed with me and took more than five years of incredible collaborative work to answer it."
Until now it was believed that these fans were powered solely by muscle action. However, a study published today in Science, reports that Rhagovelia's flat, ribbon-shaped fans can instead passively morph using surface tension and elastic forces, without relying on muscle energy.
"Observing for the first time an isolated fan passively expanding almost instantaneously upon contact with a water droplet was entirely unexpected," said Dr. Ortega-Jimenez.
This remarkable combination of collapsibility during leg recovery and rigidity during propulsion allows the bugs to execute sharp turns in just 50 milliseconds and move at speeds up to 120 body lengths per second, rivaling the rapid aerial maneuvers of flying flies.
Collaboration is key
When Dr. Ortega-Jimenez joined Georgia Tech in 2020 after leaving KSU, he presented the project and preliminary observations on Rhagovelia bugs to Dr. Saad Bhamla, who became fascinated and eager to explore it further. It was Dr. Bhamla who brought Dr. Je-Sung's group into the collaboration, opening new possibilities to integrate biology, physics, and robotics into the project.
"I saw a real discovery hiding in plain sight. Often, we think science is a lone genius sport, but this couldn't be farther from the truth. Modern science is all about interdisciplinary team of curious scientists working together, across borders and disciplines to study nature and engineer new bioinspired machines" Said Dr Bhamla
This interdisciplinary effort, integrating experimental biology, fluid physics, and engineering design, continued for more than five years.
Rhagobot is born: The next generation of water strider robots
Creating an insect-size robot inspired by ripple bugs was a major challenge, particularly because the microstructural design of the fan remained a mystery. The breakthrough came when Dr. Dongjin Kim and Professor Je-Sung from Ajou University captured high-resolution images of the fan using a scanning electron microscope, that they were able to uncover the solution to this puzzle.
"We initially designed various types of cylindrical-shaped fans, which we generally think what hair looks like. However, the functional duality of the fan—rigidity for thrust generation and flexible for collapsibility—could not be achieved with cylindrical structures. After numerous attempts, we overcame this challenge by designing a flat-ribbon shaped fan. We strongly suspected that biological fans might share a similar morphology, and eventually discovered that the Rhagovelia fan indeed possess a flat-ribbon micro architecture, which had not been previously reported. This discovery further validated the design principle behind our artificial flat-ribbon fan." said Dr Dongjin Kim, a postdoctoral researcher at Ajou University and also a lead author of this study.
With these insights they were able to decode the structural basis and function of this natural propulsion system and recreate it in a robotic form. The result was the engineering of a one milligram elastocapillary fan that deploys itself, which was integrated into an insect-size robot. This microrobot is capable of enhanced thrust, braking, and maneuverability, validated through experiments involving both live insects and robotic prototypes.
"Our robotic fans self-morph using nothing but water surface forces and flexible geometry—just like their biological counterparts. It is a form of mechanical embedded intelligence refined by nature through millions of years of evolution. In small-scale robotics, these kinds of efficient and unique mechanisms would be a key enabling technology for overcoming limits in miniaturization of conventional robots." said Professor Je-sung Koh, a senior author of the study.
The study not only establishes a direct link between fan microstructure and aquatic locomotion control, but also lays the foundation for future design of compact, semi-aquatic robots that can explore water surfaces in challenging, fast-flowing environments.
The ripple bug's fan structure, which rapidly collapses and reopens as it enters and exits water, has revealed an unprecedented biomechanical duality—high flexibility for rapid deployment and high rigidity for thrust. This duality addresses longstanding limitations in small-scale aquatic robotics, such as inefficient stroke recovery and limited maneuvering capacity.
Sketching vortices and waves on water
It is well known that during propulsion, non-fanned water striders (e.g., those of the Gerridae family) generate characteristic dipolar vortices and capillary waves when stroking their superhydrophobic legs on the water. In contrast, fanned Rhagovelia bugs produce a distinct and complex vortical signature with each stroke, closely resembling the wake produced by flapping wings in air.
"It's as if Rhagovelia have tiny wings attached to their legs, like the Greek god Hermes" said Dr. Ortega-Jimenez. "Future research is needed to determine whether ripple bugs can similarly produce lift-based thrust with their fan-like structures, in addition to drag-based propulsion."
This possibility is intriguing, because evidence suggests that whirligig beetles and cormorants generate hydrodynamic lift for swimming propulsion via their hairy legs and webbed feet, respectively.
In addition to these vortices, Rhagovelia bugs also produce symmetrical capillary waves during leg propulsion, which appear to aid in thrust generation, along with strong bow waves that form at the front of the body.
Standing against turbulent waters
Natural streams pose a real challenge, especially for tiny animals that live and move at the interface. Ripple bugs, roughly the size of a grain of rice, must navigate highly dynamic, wavy, and turbulent waters, while escaping predators, catching prey and finding mates. The relative levels of turbulence that these insects endure daily far exceed what we typically experience during airplane turbulence. Surprisingly, twenty-four-hour monitoring of these bugs in the lab revealed their remarkable endurance.
"They literally row day and night throughout their lifespan, only pausing to molt, mate, or feed," said Ortega-Jimenez.
These unsteady conditions found in streams represent also a significant difficulty for designing interfacial micro-robots capable of moving effectively across such unpredictable waters.
"When designing small-scale robots, it's important to account for the specific environment in which they will operate—in this case, the water's surface. By leveraging the unique properties of that environment, a robot's performance and efficiency can be greatly enhanced. The Rhagobot, for instance, can travel quickly along a flowing stream thanks to its intelligent fan structure, which is powered by surface tension and the drag forces from the water surface." said Jesung Koh.
Finally, these discoveries can have wide-ranging implications for bioinspired robotics, particularly in the development of environmental monitoring systems, search-and-rescue microrobots, and devices capable of navigating perturbed water-air interfaces with insect-like dexterity.
